1. Field
The present application relates generally to substrate fabrication and more particularly, to a method and apparatus for using multiple source chemical vessels to deliver the same source chemical in a vapor deposition process.
2. Description of the Related Art
In the fabrication of integrated circuits on substrates, such as semiconductor wafers, the vapor deposition of chemicals, such as chemical vapor deposition (“CVD”) and atomic layer deposition (“ALD”), is often desirable. The expansion of suitable source chemicals has increasingly led to the use of precursor materials that are naturally liquid or solid at room temperature and atmospheric pressures.
Typically, an ALD process employs alternating surface reactions whereby a substrate on which deposition is to be conducted is maintained at a temperature above the condensation temperature for the vapor phase reactants and below the thermal decomposition temperatures. ALD is a chemically self-limiting process in which alternating pulses of source chemicals saturate the substrate and leave no more than a monolayer of material per pulse. The source chemicals are selected to ensure self-saturating reactions because at least one pulse per cycle leaves a chemisorbed species with a surface termination that is non-reactive with the gas phase reactants of the same pulse. Such a termination is left by “tails” or ligands of the source chemical, such as organic tails or halide tails. A subsequent pulse of a different reactant reacts with the tails left on the chemisorbed layer of the previous pulse to enable continued deposition. Thus, each cycle of alternated pulses leaves no more than about one monolayer of the desired material on the substrate. The principles of ALD type processes have been presented by T. Suntola, e.g., in the Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth mechanisms and Dynamics, Chapter 14, Atomic Layer Epitaxy, pp. 601-663, Elsevier Science B.V. 1994.
ALD facilitates the formation of thin films monolayer by monolayer. The skilled artisan will understand that control exists on a smaller than monolayer scale, due to steric hindrance of bulk source chemical molecules producing less than one monolayer per cycle. The capability of layering atomically thin monolayers enables forming more precise concentration gradients from the lower surface, for example, at the gate dielectric/silicon substrate interface, to the upper surface, for example, at the gate electrode/gate dielectric interface.
Accordingly, each discrete monolayer, or partial monolayer, can be tailored by selectively introducing the desired chemistry for each monolayer to be deposited. For example, by means of ALD, a particular combination of introduced gases react with, deposit or adsorb on the substrate until, by the nature of the deposition chemistry itself, the process self-terminates. Regardless of the length of exposure, the process gases do not contribute significantly to further deposition. To deposit subsequent monolayers, different chemical combinations are introduced into the process chamber that will react with or adsorb on the previously formed monolayer. Desirably, the second chemistry or a subsequent chemistry forms another monolayer, also in a self-limiting manner. These self-limiting monolayers are alternated as many times, by repeating cycles, as desired to form a film of suitable thickness.
Each cycle of an ALD process includes a plurality of pulses. In an ALD process, sequential reactant pulses of two different source chemicals are separated in both time and space to avoid gas phase reaction, as the reactants are typically highly mutually reactive, as described above. For example, after a first reactant pulse, excess reactant and by-product is purged from the reaction chamber with an inert gas, such as nitrogen. In a first pulse, the reactant adsorbs on the substrate in a self-saturating process, leaving no more than about one monolayer of reactant. The reactant typically includes termination ligands that are not reactive with the gas phase of the same reactant pulse. After purging with an inert gas, a second reactant pulse take place and the second reactant reacts with the termination ligands, either stripping or replacing the ligands with another element or molecule of interest for the deposited layer. Excess of the second reactant and by-products are then purged and the cycle starts again with the first reactant, or, alternatively, a third reactant can be introduced. Accordingly, no more than a molecular monolayer of the desired material is formed per cycle. In fact, typically less than a monolayer will form, due to steric hindrance by bulky source chemicals blocking possible reactive sites on the surface or due to limited number of reactive sites on the surface.
In accordance with general trends in semiconductor manufacturing, integrated circuits are continually being scaled down in pursuit of faster processing speeds and lower power consumption. As die area for each device decreases with each technology generation, some circuit designs are using more structures with high aspect ratio features in order to better use available chip area. For example, certain dynamic random access memory (DRAM) capacitors employ deep trenches. Such trenches can be very narrow and deep, having aspect ratios of 40:1 or greater. As the packing density of devices increases, each semiconductor device must still meet certain requirements. For example, each DRAM capacitor must still maintain a certain minimum charge storage to ensure reliable operation of the memory cell without excessive refresh cycling. Further, future DRAM trench capacitors require high-k dielectric films, which are more effective when the films are conformally formed. Other examples of devices requiring high step coverage or high aspect ratio features include microelectromechanical systems (MEMS) devices, in which surfaces to be coated often entail reaching through holes to cavities with reentrant profiles, such as MEMS pressure sensors, microfluidic ejection heads, etc.
As used herein, conformality refers to substantially complete coverage of a target surface. However, it is not straightforward to uniformly deposit materials directly over high aspect ratio structures in order to create thin films that meet certain specifications for a desired application.
ALD processes, similar to those described above, have yielded inconsistent results in forming conformal thin films directly over high aspect ratio features. For example, as presented in Schroeder et. al., “Recent Developments in ALD Technology For 50 nm Trench DRAM Applications,” ECS Transactions, 1 (5), pages 125-132 (2006) 10.1149/1.2209261 (hereinafter referred to as “Schroeder”), ALD growth of HfO2 (hafnium oxide) from precursors HfCl4 (hafnium chloride) and H2O (water) was not successful in achieving acceptable “step coverage” in high aspect ratio trenches, while ALD processes with TEMAH (Tetrakis ethyl methyl amino hafnium, also referred to as “TEMAHf” or “TEMAH”) and O3 (ozone) produced better results. As used herein, step coverage of a substrate feature having a given aspect ratio, such as a trench, refers to the ratio of deposited film thickness at the bottom of the feature to the deposited film thickness on the top surface of the substrate. It is understood that the bottom of a feature has a deposited film thickness that is typically less than or equal to that of the top surface of the substrate, and that a high step coverage is typically less than but close to 1.0. Schroeder hypothesized that the poor step coverage achieved from the use of HfCl4 and H2O was due to high or low sticking probability of the precursors.
ALD has been used to form thin films directly over trenches in DRAM capacitors, as disclosed in U.S. Pat. No. 6,780,704 issued to Raaijmakers et al. This prior art achieved high step coverage in creating a thin film over a DRAM capacitor during an ALD process operating at a temperature of 150-350° C.
FIG. 1 illustrates undesirable results in attempting to form a conformal thin film directly over a high aspect ratio feature, such as efforts to form HfO2 from HfCl4 and H2O in Schroeder. The substrate 166 contains a deep, narrow trench 190 with a thin film 192 achieving 0% step coverage. A skilled artisan will recognize FIG. 1 as exhibiting depletion effects more typically associated with CVD or PVD. In the lower part of the trench 190, there is no thin film to serve as a dielectric, which will not result in a usable DRAM capacitor.
Even some successful processes, such as ALD with precursors TEMAH and O3, have undesirable limitations. For example, TEMAH decomposes at high temperatures, exhibiting deposition and depletion effects sometimes associated with CVD, instead of self-limiting ALD surface reactions and uniform deposition over high aspect ratio features. This decomposition can be minimized or prevented by lowering the temperature of the precursor source vessel, delivery system, and reactor temperature. For example, for TEMAH, the precursor delivery system temperature can be maintained at about 90-150° C. and the reactor temperature can be lowered to about 200-300° C., depending on reactor design. However, these limitations on temperature have drawbacks. Lowering the temperature of the precursor delivery system can reduce the efficiency of bubblers or vaporizers, and lowering the reactor temperature often results in lower quality films with more contaminants and higher leakage current.
Even for applications where the aspect ratio is not extreme, it can sometimes be challenging to supply vapor phase reactant consistently pulse after pulse (for ALD) or wafer after wafer (more generally), particularly where a high volume of the reactant is needed, such as when each wafer receives a very thick film and/or a very large surface needs to be coated. Examples include large format deposition tools for flat screen applications, batch deposition reactors or roll-to-roll coating on flexible substrates.
Accordingly, there is a need for achieving high film quality in an economical way while producing high quality semiconductor devices and associated structures.